Congrats on the achievement. A metric century isn't much more, I would guess you'd be able to do it in another month.

Do you have a cadence meter? I've noticed on longer riders, especially if it's a new distance, we have a tendency to drop our cadence when we start to get fatigued. Cadence in the 60s and low 70s can cause knee issues for some people (myself included) for longer rides. Another thing is if it isn't just distance or cadence related, you might adjust your cleat so your foot is neutral vs. bowed in since you reported pain on the outer knee. But first I'd check your cadence, finding a resolution to knee pain can be a long and arduous process.